Method for obtaining a printing plate according to the silver salt diffusion transfer process

ABSTRACT

The present invention provides a method for making a lithographic printing plate according to the silver salt diffusion transfer process using a photographic material comprising on a support a silver halide emulsion layer and a layer containing physical development nuclei comprising the steps of: 
     image-wise exposing said photographic material; 
     developing a thus obtained image-wise exposed photographic material by guiding said image-wise exposed photographic material through an alkaline processing liquid comprising a hydrophobizing agent, in 15s or less and 
     subsequently guiding a thus obtained developed photographic material through a stabilizing liquid; 
     characterized in that said stabilizing liquid comprises a compound having a mercapto-group having a pK a  of 4.5 or less.

FIELD OF THE INVENTION

The present invention relates to a method for making a lithographicprinting plate according to the silver salt diffusion transfer process.

BACKGROUND OF THE INVENTION

The principles of the silver complex diffusion transfer reversalprocess, hereinafter called DTR-process, have been described e.g. inU.S. Pat. No. 2,352,014 and in the book "Photographic Silver HalideDiffusion Processes" by Andre Rott and Edith Weyde--The FocalPress--London and New York, (1972).

In the DTR-process non-developed silver halide of an information-wiseexposed photographic silver halide emulsion layer material istransformed with a so-called silver halide solvent into soluble silvercomplex compounds which are allowed to diffuse into an image-receivingelement and are reduced therein with a developing agent, generally inthe presence of physical development nuclei, to form a silver imagehaving reversed image density values ("DTR-image") with respect to theblack silver image obtained in the exposed areas of the photographicmaterial.

A DTR-image bearing material can be used as a planographic printingplate wherein the DTR-silver image areas form the water-repellantink-receptive areas on a water-receptive ink-repellant background. Aparticular interesting type of photographic material for making aprinting plate according to the DTR-process comprises in the order givenon a support a silver halide emulsion layer and a physical developmentnuclei containing layer as a surface layer. Such type of photographicmaterial is commercially available under the tradename of SUPERMASTERfrom Agfa-Gevaert NV and from Mitsubishi Paper Mills under the tradenameSILVERMASTER.

These commercial printing plates are generally processed by means ofcamera plate makers that include an exposure unit as well as adeveloping unit. As a result of a demand on the market to reduce thetime needed for producing a printing plate, the processing times in suchcamera plate makers have been shorted.

In particular the time that lapses between entrance of the photographicmaterial into the developing liquid and exit of the photographicmaterial out of the developing liquid is substantially reduced.Typically the time has been reduced to less than 15s and may even beless than 10s.

It has now been found by us that the automatic processing of aphotographic material according to the DTR-process in a camera platemaker that uses short development times of 15s or less, results inprinting plates of inferior quality. In particular, it was found thatwhen the plates were stored for some time after plate-making but beforeactual printing, the printing endurance was much less than in case theprinting plate was used to print immediately after plate-making.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for makinga printing plate according to the silver salt diffusion transfer processwith reduced dipping times into the developing liquid and which printingplate can be stored without substantial loss of the printing qualitysuch as printing endurance and ink-acceptance.

Further objects of the present invention will become clear from thedescription hereinafter.

According to the present invention there is provided a method for makinga lithographic printing plate according to the silver salt diffusiontransfer process using a photographic material comprising on a support asilver halide emulsion layer and a layer containing physical developmentnuclei comprising the steps of:

image-wise exposing said photographic material;

developing a thus obtained image-wise exposed photographic material byguiding said image-wise exposed photographic material through analkaline processing liquid comprising a hydrophobizing agent, in 15s orless and

subsequently guiding a thus obtained developed photographic materialthrough a stabilising liquid;

characterised in that said stabilising liquid comprises a compoundhaving a mercapto-group having a pK_(a) of 4.5 or less.

DETAILED DESCRIPTION

According to the present invention it has been found that the problemsof loss of printing quality during storage of a printing plate obtainedaccording to the DTR-process using short dipping times can be overcomeby using a stabilising liquid that comprises a compound having amercapto-group having a pK_(a) of 4.5 or less. It is essential to thepresent invention that a compound be used that has a mercapto-group ofpK_(a) -value of 4.5 or less. When using a compound having only amercapto-group of pK_(a) -value of more than 4.5, the printing enduranceof the plate during storage may be kept but in that case it is observedthat the ink-acceptance of the plate decreases resulting in a much morecopies that have to be disposed of during start-up of the print job.

Using a stabilising liquid in accordance with the present invention evenallows dipping times of 10s or less.

The pK_(a) of the mercapto-compounds in connection with the presentinvention were determined according to the method described in"Ionization Constants of Acids and Bases--A Laboratory Manual" writtenby Adrien Albert and E. P. Serjeant and Edited by John Wiley & Sons (NewYork) page 110-112 (1962).

In these particular cases, the compounds were dissolved in water at pH11. The concentration of the compounds was 0.002 mole/liter and themixture was then titrated with a solution of hydrochloric acid (0.1mole/liter).

Mercapto-compounds that are suitable for use in connection with thepresent invention are compounds having a mercapto group with a pK_(a) of4.5 or less, preferably of a pK_(a) of 4 or less, more preferably 3.7 orless and that are preferably substituted with a group containing atleast 3 carbons and more preferably at least 5 carbon atoms. Preferablythe compounds are heterocyclic compounds containing the mercapto groupon the heterocyclic ring. In a highly preferred embodiment in connectionwith the present invention, the mercapto-compounds are hydrophobizingagents capable of improving the ink-acceptance of the silver image.

Particularly preferred compounds for use in connection with the presentinvention correspond to the following formula (I): ##STR1## wherein Zrepresents a substituted or unsubstituted alkyl, a substituted orunsubstituted aryl, an aralkyl, an alkylaryl, a substituted orunsubstituted alkenyl, a substituted or unsubstituted alkynyl or aheterocyclic ring.

Specific examples of compounds according to formula (I) are listed intable 1.

                  TABLE 1                                                         ______________________________________                                        compound no.   Z in formula (I)                                                                           pK.sub.a                                          ______________________________________                                        1              CH.sub.2 CONH-n.C.sub.6 H.sub.13                                                           3.3                                               2              CH.sub.2 CON(n.C.sub.4 H.sub.9).sub.2                                                      3.3                                               3              n.C.sub.6 H.sub.13                                                                         3.4                                               4              n.C.sub.8 H.sub.17                                                                         3.4                                               5              n.C.sub.4 H.sub.9                                                                          3.2                                               6              C.sub.6 H.sub.5                                                                            3.1                                               ______________________________________                                    

According to the present invention the mercapto-compounds may becontained in the stabilising liquid in an amount of at least 10⁻⁴ mol/l,more preferably at least 10⁻³ mol/l. The maximum amount of themercapto-compound will be determined by the type of compound, type andamount of silver halide solvents etc. Typically the concentration ofmercapto-compound is preferably not more than 10⁻¹ mol/l.

The stabilising liquid generally has a pH between 5 and 7. Thestabilising liquid preferably contains a buffer e.g. a phosphate buffer,a citrate buffer or mixture thereof and preferably an alkanol amine suchas the alkanol amines listed furtheron. The buffer capacity of thestabilising liquid is preferably such that the addition of 0.5 ml of a 1molar aqueous sodium hydroxide solution to 50 ml of stabilising liquiddoes not increase the pH of the stabilising above 6.5. The stabilisingliquid can further contain bactericides, e.g. phenol, thymol or5-bromo-5-nitro-1,3-dioxan as described in EP 0,150,517. The liquid canalso contain substances which influence the hydrophobic/hydrophilicbalance of the printing plate obtained after processing of the DTRelement, e.g. silica. Further the stabilising liquid can contain wettingagents, e.g. pluronic and preferably compounds (wetting agents)containing perfluorinated alkyl groups.

The hydrophobizing agents for use in the alkaline processing liquid arecompounds that are capable of reacting with silver or silver ions andthat are hydrophobic i.e. insoluble in water or only slightly soluble inwater. Generally these compounds contain a mercapto group or thiolategroup and one or more hydrophobic substituents e.g. an alkyl containingat least 3 carbon atoms. Examples of hydrophobizing agents for use inaccordance with the present invention are e.g. those described in U.S.Pat. Nos. 3,776,728, and 4,563,410. Preferred compounds correspond toone of the following formulas or correspond to formula (I) shown above:##STR2## wherein R⁵ represents hydrogen or an acyl group, R⁴ representsalkyl, aryl or aralkyl. Most preferably used compounds are compoundsaccording to one of the above formulas wherein R⁴ represents an alkylcontaining 3 to 16 C-atoms.

According to the present invention the hydrophobizing agents arecontained in the alkaline processing liquid in an amount of at least 0.1g/l, more preferably at least 0.2 g/l and most preferably at least 0.3g/l. The maximum amount of hydrophobizing agents will be determined bythe type of hydrophobizing agent, type and amount of silver halidesolvents etc. Typically the concentration of hydrophobizing agent ispreferably not more than 1.5 g/l and more preferably not more than 1g/l.

According to a preferred embodiment the alkaline processing solutionalso contains a meso ionic compound. Meso-ionic compounds as referred toin the present invention are a group of compounds defined by W. Bakerand W. D. Ollis as "5- or 6-membered heterocyclic compounds which cannotbe represented satisfactorily by any one covalent or polar structure andpossesses a sextet of π-electrons in association with the atomscomprising the ring. The ring bears a fractional positive chargebalanced by a corresponding negative charge located on a covalentlyattached atom or group of atoms" as described in Quart. Rev., Vol. 11,p. 15 (1957) and Advances in Heterocyclic Chemistry, Vol. 19, P. 4(1976).

Preferred meso-ionic compounds are those represented by formula (IV):##STR3## wherein M represents a 5- or 6-membered heterocyclic ringcomposed of at least one member selected from the group consisting of acarbon atom, a nitrogen atom, an oxygen atom, a sulfur atom and aselenium atom; and A⁻ represents --O⁻, --S⁻ or --N⁻ --R, wherein Rrepresents an alkyl group (preferably having 1 to 6 carbon atoms), acycloalkyl group (preferably having 3 to 6 carbon atoms), an alkenylgroup (preferably having 2 to 6 carbon atoms) an alkynyl group(preferably having 2 to 6 carbon atoms), an aralkyl group, an aryl group(preferably having 6 to 12 carbon atoms), or a heterocyclic group(preferably having 1 to 6 carbon atoms).

In formula (IV), examples of the 5-membered heterocyclic ring asrepresented by M include an imidazolium ring, a pyrazolium ring, anoxazolium ring, an isoxazolium ring, a thiazolium ring, an isothiazoliumring, a 1,3-dithiol ring, a 1,3,4- or 1,2,3-oxadiazolium ring, a1,3,2-oxathiazolium ring, a 1,2,3-triazolium ring, a 1,3,4-triazoliumring, a 1,3,4-, 1,2,3- or 1,2,4-thiadiazolium ring, a1,2,3,4-oxatriazolium ring, a 1,2,3,4-tetrazolium ring and a1,2,3,4-thiatriazolium ring.

Preferred meso-ionic compounds for use in accordance with the presentinvention are triazolium thiolates and more preferably1,2,4-triazolium-3-thiolates and most preferably those that correspondto the following formula: ##STR4## wherein R⁷ and R⁸ each independentlyrepresents an unsubstituted or substituted alkyl group, alkenyl group,cycloalkyl group, aralkyl group, aryl group or heterocyclic group, Arepresents an unsubstituted or substituted alkyl group, alkenyl group,cycloalkyl group, aralkyl group, aryl group, heterocyclic group or --NR⁹R¹⁰ wherein R⁹ and R¹⁰ each independently represents hydrogen, an alkylgroup or aryl group or R⁷ and R⁸ or R⁸ and A or R⁹ and R¹⁰ represent thenecessary atoms to form a 5- or 6-membered ring.

Specific examples of 1,2,4-triazolium-3-thiolates suitable for use inaccordance with the present invention are shown in table 2.

                  TABLE 2                                                         ______________________________________                                         ##STR5##             (1)                                                      ##STR6##             (2)                                                      ##STR7##             (3)                                                      ##STR8##             (4)                                                      ##STR9##             (5)                                                      ##STR10##            (6)                                                      ##STR11##            (7)                                                      ##STR12##            (8)                                                      ##STR13##            (9)                                                     ______________________________________                                    

Preferably the amount of meso-ionic compound in the alkaline processingliquid is between 0.1 mmol/l and 50 mmol/l and more preferably between0.1 mmol/l and 25 mmol/l and most preferably between 0.5 mmol/l and 10mmol/l.

Preferably, the alkaline processing solution also contains analkanolamine. Alkanolamines that are suitable for use in accordance withthe present invention may be of the tertiary, secondary or primary type.Examples of alkanolamines that may be used in connection with thepresent invention correspond to the following formula: ##STR14## whereinX and X' independently represent hydrogen, a hydroxyl group or an aminogroup, k and m represent 0 or integers of 1 or more and n represents aninteger of 1 or more. Preferably used alkanolamines are e.g.N-(2-aminoethyl)ethanolamine, diethanolamine, N-methylethanolamine,triethanolamine, N-ethyldiethanolamine, diisopropanolamine,ethanolamine, 4-aminobutanol, N,N-dimethylethanolamine, 3-aminopropanol,N,N-ethyl-2,2'-iminodiethanol etc. or mixtures thereof.

The alkanolamines are preferably used in a concentration of 0.05% to 10%by weight and more preferably in a concentration of 0.05% by weight to7% by weight.

In addition to the hydrophobizing compound and optionally a meso-ioniccompound and/or an alkanolamine the alkaline processing liquid canfurther comprise silver halide solvents. Additional silver halidesolvents for use in connection with the present invention are e.g.2-mercaptobenzoic acid, cyclic imides, oxazolidones, thiocyanates,thioethers and thiosulfates, polyamines e.g. ethylene diamine,n-propylene diamine, n-butylene diamine or such polyamines as disclosedin Japanese Laid-Open patent publication no. 06-222560.

Preferably used thioethers correspond to the following general formula:

    Z--(R.sup.1 --S).sub.t --R.sup.2 --S--R.sup.3 --Y

wherein Z and Y each independently represents hydrogen, an alkyl group,an amino group, an ammonium group, a hydroxyl, a sulfo group, acarboxyl, an aminocarbonyl or an aminosulfonyl, R¹, R² and R³ eachindependently represents an alkylene that may be substituted andoptionally contain a oxygen bridge and t represents an integer from 0 to10.

Examples of thioether compounds corresponding to the above formula aredisclosed in e.g. U.S. Pat. No. 4,960,683 and specific examples arelisted in table 3.

                  TABLE 3                                                         ______________________________________                                        (HOCH.sub.2 CH.sub.2).sub.2 S                                                                            1                                                  HOCH.sub.2 CH.sub.2 SCH.sub.2 SCH.sub.2 CH.sub.2 OH                                                      2                                                  HOCH.sub.2 CH.sub.2 S(CH.sub.2).sub.2 SCH.sub.2 CH.sub.2 OH                                              3                                                  HOCH.sub.2 CH.sub.2 S(CH.sub.2).sub.3 SCH.sub.2 CH.sub.2 OH                                              4                                                  HOCH.sub.2 CH.sub.2 S(CH.sub.2).sub.4 SCH.sub.2 CH.sub.2 OH                                              5                                                  C.sub.2 H.sub.5 SCH.sub.2 CH.sub.2 SCH.sub.2 CHOHCH.sub.2 OH                                             6                                                  (HOCH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2).sub.2 O                                                         7                                                  HOCH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 N(CH.sub.3).sub.2                                                 8                                                  (HOCH.sub.2 CHOHCH.sub.2 SCH.sub.2).sub.2                                                                9                                                  HOOCCH.sub.2 SCH.sub.2 SCH.sub.2 COOH                                                                    10                                                 HOOCCH.sub.2 S(CH.sub.2).sub.2 SCH.sub.2 COOH                                                            11                                                 HOOCCH.sub.2 S(CH.sub.2).sub.3 SCH.sub.2 COOH                                                            12                                                 HOOC(CH.sub.2).sub.2 SCH.sub.2 S(CH.sub.2).sub.2 COOH                                                    13                                                 HOOC(CH.sub.2).sub.2 S(CH.sub.2).sub.2 S(CH.sub.2).sub.2 COOH                                            14                                                 HOOC(CH.sub.2).sub.2 S(CH.sub.2).sub.3 S(CH.sub.2).sub.2 COOH                                            15                                                  ##STR15##                 16                                                 ______________________________________                                    

Preferably the silver halide solvent is used in an amount between 0.05%by weight and 10% by weight and more preferably between 0.05% by weightand 7% by weight.

The alkaline processing liquid used in accordance with the presentinvention preferably has a pH between 9 and 14 and more preferablybetween 10 and 13. Said pH may be established by an organic or inorganicalkaline substance or a combination thereof. Suitable inorganic alkalinesubstances are e.g. potassium or sodium hydroxide, carbonate, phosphateetc. Suitable organic alkaline substances are e.g. alkanolamines. In thelatter case the alkanolamines will provide or help maintain the pH andserve as a silver halide complexing agent. The present invention allowsthe use of alkaline processing liquids of low pH (11 or less) to beused. This is a further advantage of the present invention because suchliquids are less corrosive and less susceptible to exhaustion.

The alkaline processing liquid may also contain (a) developing agent(s).In this case the alkaline processing liquid is called a developer. Onthe other hand some or all of the developing agent(s) may be present inone or more layers of the photographic material or imaging element. Whenall of the developing agents are contained in the imaging element thealkaline processing liquid is called an activator or activating liquid.

Silver halide developing agents for use in accordance with the presentinvention are e.g. ascorbic acid, of the p-dihydroxybenzene type, e.g.hydroquinone, methylhydroquinone or chlorohydroquinone, preferably incombination with an auxiliary developing agent being a1-phenyl-3-pyrazolidinone-type developing agent and/orp-monomethylaminophenol. Particularly useful auxiliary developing agentsare of the phenidone type e.g. 1-phenyl-3-pyrazolidinone,1-phenyl-4-monomethyl-3-pyrazolidinone, and1-phenyl-4,4-dimethyl-3-pyrazolidinone. Preferred phenidone typedeveloping agents, particularly when they are incorporated into thephotgraphic material are phenidones of which the aqueous solubility isincreased by a hydrophilic substituent such as e.g. hydroxy, amino,carboxylic acid group, sulphonic acid group etc. Examples of phenidonessubsituted with one or more hydrophilic groups are e.g.1-phenyl-4,4-dimethyl-2-hydroxy-3-pyrazolidone,1-(4-carboxyphenyl)-4,4-dimethyl-3-pyrazolidone etc. However otherdeveloping agents can be used.

At least the auxiliary developing agents are preferably incorporatedinto the photographic material. The auxiliary developing agent ispreferably incorporated in the photographic material, preferably in thesilver halide emulsion layer of the photographic material, in an amountof less than 150 mg/g of silver halide expressed as AgNO₃, morepreferably in an amount of less than 100 mg/g of silver halide expressedas AgNO₃.

The alkaline processing liquid preferably also contains a preservingagent having antioxidation activity, e.g. sulphite ions provided e.g. bysodium or potassium sulphite. For example, the aqueous alkaline solutioncomprises sodium sulphite in an amount ranging from 0.15 to 1.0 mol/l.Further may be present a thickening agent, e.g. hydroxyethylcelluloseand carboxymethylcellulose, fog inhibiting agents, e.g. potassiumbromide, potassium iodide and a benzotriazole which is known to improvethe printing endurance, calcium-sequestering compounds, anti-sludgeagents, and hardeners including latent hardeners. In accordance with thepresent invention it is furthermore preferred to use a spreading agentor surfactant in the alkaline processing liquid to assure equalspreading of the alkaline processing liquid over the surface of thephotographic material. Such a surfactant should be stable at the pH ofthe alkaline processing liquid and should assure a fast overall wettingof the surface of the photographic material. A surfactant suitable forsuch purpose is e.g. a fluor containing surfactant such as e.g. C₇ F₁₅COONH₄. It is furthermore advantageous to add glycerine to the alkalineprocessing liquid so as to prevent crystallization of dissolvedcomponents of said alkaline processing liquid.

Development acceleration can be accomplished with the aid of variouscompounds to the alkaline processing liquid and/or one or more layers ofthe photographic element, preferably polyalkylene derivatives having amolecular weight of at least 400 such as those described in e.g. U.S.Pat. Nos. 3,038,805--4,038,075--4,292,400--4,975,354.

Shortly before printing the plate may be wiped with a so-called etchsolution. Conveniently this etch solution has a composition similar tothe stabilising liquid.

The image-wise exposure in connection with the present invention may bea camera exposure but may also be a scan-wise exposure e.g. by means ofa laser or LED.

A photographic material for use in connection with the method of thepresent invention comprises on a support in the order given a silverhalide emulsion layer and a layer containing physical developmentnuclei.

Supports suitable for use in accordance with the present invention maybe opaque or transparent, e.g. a paper support or resin support. When apaper support is used preference is given to one coated at one or bothsides with an Alpha-olefin polymer, e.g. a polyethylene layer whichoptionally contains an anti-halation dye or pigment. It is also possibleto use an organic resin support e.g. cellulose nitrate film, celluloseacetate film, poly(vinyl acetal) film, polystyrene film, poly(ethyleneterephthalate) film, polycarbonate film, polyvinylchloride film orpoly-Alpha-olefin films such as polyethylene or polypropylene film. Thethickness of such organic resin film is preferably comprised between0.07 and 0.35 mm. These organic resin supports are preferably coatedwith a hydrophilic adhesion layer which can contain water insolubleparticles such as silica or titanium dioxide. Metal supports e.g.aluminium may also be used in accordance with the present invention.

The image receiving layer containing physical development nuclei ispreferably free of hydrophilic binder but may comprise small amountsupto 30% by weight of the total weight of said layer of a hydrophiliccolloid e.g. polyvinyl alcohol to improve the hydrophilicity of thesurface. Preferred development nuclei for use in accordance with thepresent invention are sulphides of heavy metals e.g. sulphides ofantimony, bismuth, cadmium, cobalt, lead, nickel, palladium, platinum,silver, and zinc. Especially suitable development nuclei in connectionwith the present invention are palladium sulphide nuclei. Other suitabledevelopment nuclei are salts such as e.g. selenides, polyselenides,polysulphides, mercaptans, and tin (II) halides. Heavy metals,preferably silver, gold, platinum, palladium, and mercury can be used incolloidal form.

The photographic silver halide emulsion(s) used in accordance with thepresent invention can be prepared from soluble silver salts and solublehalides according to different methods as described e.g. by P. Glafkidesin "Chimie et Physique Photographique", Paul Montel, Paris (1967), by G.F. Duffin in "Photographic Emulsion Chemistry", The Focal Press, London(1966), and by V. L. Zelikman et al in "Making and Coating PhotographicEmulsion", The Focal Press, London (1966).

The photographic silver halide emulsions used according to the presentinvention can be prepared by mixing the halide and silver solutions inpartially or fully controlled conditions of temperature, concentrations,sequence of addition, and rates of addition. The silver halide can beprecipitated according to the single-jet method or the double-jetmethod.

The silver halide particles of the photographic emulsions used accordingto the present invention may have a regular crystalline form such as acubic or octahedral form or they may have a transition form. They mayalso have an irregular crystalline form such as a spherical form or atabular form, or may otherwise have a composite crystal form comprisinga mixture of said regular and irregular crystalline forms.

According to the present invention the emulsion or emulsions preferablyconsist principally of silver chloride while a fraction of silverbromide may be present ranging from 1 mole % to 40 mole %. Mostpreferably the amount of bromide is kept below 5 mole %. The emulsionsmay further contain silver iodide in an amount of upto 5 mole %preferably upto 2 mole %.

The average size of the silver halide grains may range from 0.10 to 0.70μm , preferably from 0.25 to 0.45 μm.

The size distribution of the silver halide particles of the photographicemulsions to be used according to the present invention can behomodisperse or heterodisperse. A homodisperse size distribution isobtained when 95% of the grains have a size that does not deviate morethan 30% from the average grain size.

Preferably during the precipitation stage Iridium and/or Rhodiumcontaining compounds or a mixture of both are added. The concentrationof these added compounds ranges from 10⁻⁸ to 10⁻³ mole per mole ofAgNO₃, preferably between 10⁻⁷ and 10⁻⁶ mole per mole of AgNO₃. Thisresults in the building in in the silver halide crystal lattice of minoramounts of Iridium and/or Rhodium, so-called Iridium and/or Rhodiumdopants. As known to those skilled in the art numerous scientific andpatent publications disclose the addition of Iridium or Rhodiumcontaining compounds or compounds containing other elements of GroupVIII of the Periodic System during emulsion preparation.

The emulsions can be chemically sensitized e.g. by addingsulphur-containing compounds during the chemical ripening stage e.g.allyl isothiocyanate, allyl thiourea, and sodium thiosulphate. Alsoreducing agents e.g. the tin compounds described in BE-P 493,464 and568,687, and polyamines such as diethylene triamine or derivatives ofaminomethane-sulphonic acid can be used as chemical sensitizers. Othersuitable chemical sensitizers are noble metals and noble metal compoundssuch as gold, platinum, palladium, iridium, ruthenium and rhodium. Thismethod of chemical sensitization has been described in the article of R.KOSLOWSKY, Z. Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951).

The emulsions of the DTR element can be spectrally sensitized accordingto the spectral emission of the exposure source for which the DTRelement is designed.

Suitable sensitizing dyes for the visible spectral region includemethine dyes such as those described by F. M. Hamer in "The Cyanine Dyesand Related Compounds", 1964, John Wiley & Sons. Dyes that can be usedfor this purpose include cyanine dyes, merocyanine dyes, complex cyaninedyes, complex merocyanine dyes, homopolar cyanine dyes, hemicyaninedyes, styryl dyes and hemioxonol dyes. Particularly valuable dyes arethose belonging to the cyanine dyes, merocyanine dyes, complexmerocyanine dyes.

The silver halide emulsions may contain the usual stabilizers e.g.homopolar or salt-like compounds of mercury with aromatic orheterocyclic rings such as mercaptotriazoles, simple mercury salts,sulphonium mercury double salts and other mercury compounds. Othersuitable stabilizers are azaindenes, preferably tetra- orpenta-azaindenes, especially those substituted with hydroxy or aminogroups. Compounds of this kind have been described by BIRR in Z. Wiss.Photogr. Photophys. Photochem. 47, 2-27 (1952). Other suitablestabilizers are i.a. heterocyclic mercapto compounds e.g.phenylmercaptotetrazole, quaternary benzothiazole derivatives, andbenzotriazole. Preferred compounds are mercapto substituted pyrimidinederivativesas disclosed in U.S. Pat. No. 3,692,527.

The silver halide emulsions may contain pH controlling ingredients.Preferably the emulsion layer is coated at a pH value below theisoelectric point of the gelatin to improve the stabilitycharacteristics of the coated layer. Other ingredients such asantifogging agents, development accelerators, wetting agents, andhardening agents for gelatin may be present. The silver halide emulsionlayer may comprise light-screening dyes that absorb scattering light andthus promote the image sharpness. Suitable light-absorbing dyes aredescribed in i.a. U.S. Pat. Nos. 4,092,168, 4,311,787 and DE-P2,453,217.

In an especially preferred embodiment the emulsion layer contained inthe imaging element contains a compound which comprises in its molecularstructure a group capable of adsorbing to silver halide and a groupcapable of reducing silver halide. Compounds of this kind have beendisclosed in EP-A-449340. In this way a combination of a stabilizing anda development activating function in one compound is achieved.

More details about the composition, preparation and coating of silverhalide emulsions can be found in e.g. Product Licensing Index, Vol. 92,December 1971, publication 9232, p. 107-109.

In addition to the above described emulsion layer and image receivinglayer other hydrophilic colloid layers in water permeable relationshipwith these layers may be present. For example it is especiallyadvantageous to include a base-layer between the support and thephotosensitive silver halide emulsion layer. In a preferred embodimentof the present invention said base-layer serves as an antihalationlayer. This layer can therefore contain the same light-absorbing dyes asdescribed above for the emulsion layer; as alternative finely dividedcarbon black can be used for the same antihalation purposes as describedin U.S. Pat. No. 2,327,828. On the other hand, in order to gainsensitivety, light reflecting pigments, e.g. titaniumdioxide can bepresent. Further this layer can contain hardening agents, mattingagents, e.g. silica particles, and wetting agents. At least part ofthese matting agents and/or light reflection pigments may also bepresent in the silver halide emulsion layer the most part howeverpreferably being present in said base-layer. As a further alternativethe light reflecting pigments may be present in a separate layerprovided between the antihalation layer and the photosensitive silverhalide emulsion layer.

In a preferred embodiment in connection with the present invention abacking layer is provided at the non-light sensitive side of thesupport. This layer which can serve as anti-curl layer can contain i.a.matting agents e.g. silica particles, lubricants, antistatic agents,light absorbing dyes, opacifying agents, e.g. titanium oxide and theusual ingredients like hardeners and wetting agents. The backing layercan consist of one single layer or a double layer pack.

The hydrophilic layers usually contain gelatin as hydrophilic colloidbinder. Mixtures of different gelatins with different viscosities can beused to adjust the theological properties of the layer. Like theemulsion layer the other hydrophilic layers are coated preferably at apH value below the isoelectric point of the gelatin. But instead of ortogether with gelatin, use can be made of one or more other naturaland/or synthetic hydrophilic colloids, e.g. albumin, casein, zein,polyvinyl alcohol, alginic acids or salts thereof, cellulose derivativessuch as carboxymethyl cellulose, modified gelatin, e.g. phthaloylgelatin etc.

The hydrophilic layers of the photographic element, especially when thebinder used is gelatin, can be hardened with appropriate hardeningagents such as those of the epoxide type, those of the ethyleniminetype, those of the vinylsulfone type e.g. 1,3-vinylsulphonyl-2-propanol,methylenebis(sulfonylethylene), chromium salts e.g. chromium acetate andchromium alum, aldehydes e.g. formaldehyde, glyoxal, and glutaraldehyde,N-methylol compounds e.g. dimethylolurea and methyloldimethylhydantoin,dioxan derivatives e.g. 2,3-dihydroxy-dioxan, active vinyl compoundse.g. 1,3,5-triacryloyl-hexahydro-s-triazine, active halogen compoundse.g. 2,4-dichloro-6-hydroxy-s-triazine, and mucohalogenic acids e.g.mucochloric acid and mucophenoxychloric acid. These hardeners can beused alone or in combination. The binders can also be hardened withfast-reacting hardeners such as carbamoylpyridinium salts of the type,described in U.S. Pat. No. 4,063,952.

Preferably used hardening agents are of the aldehyde type. The hardeningagents can be used in wide concentration range but are preferably usedin an amount of 4% to 7% of the hydrophilic colloid. Different amountsof hardener can be used in the different layers of the imaging elementor the hardening of one layer may be adjusted by the diffusion of ahardener from another layer.

The imaging element used according to the present invention may furthercomprise various kinds of surface-active agents in the photographicemulsion layer or in at least one other hydrophilic colloid layer.Suitable surface-active agents include non-ionic agents such assaponins, alkylene oxides e.g. polyethylene glycol, polyethyleneglycol/polypropylene glycol condensation products, polyethylene glycolalkyl ethers or polyethylene glycol alkylaryl ethers, polyethyleneglycol esters, polyethylene glycol sorbitan esters, polyalkylene glycolalkylamines or alkylamides, silicone-polyethylene oxide adducts,glycidol derivatives, fatty acid esters of polyhydric alcohols and alkylesters of saccharides; anionic agents comprising an acid group such as acarboxy, sulpho, phospho, sulphuric or phosphoric ester group:ampholytic agents such as aminoacids, aminoalkyl sulphonic acids,aminoalkyl sulphates or phosphates, alkyl betaines, and amine-N-oxides;and cationic agents such as alkylamine salts, aliphatic, aromatic, orheterocyclic quaternary ammonium salts, aliphatic or heterocyclicring-containing phosphonium or sulphonium salts. Preferably compoundscontaining perfluorinated alkyl groups are used. Such surface-activeagents can be used for various purposes e.g. as coating aids, ascompounds preventing electric charges, as compounds improvingslidability, as compounds facilitating dispersive emulsification and ascompounds preventing or reducing adhesion.

The photographic material of the present invention may further comprisevarious other additives such as e.g. compounds improving the dimensionalstability of the photographic element, UV-absorbers, spacing agents andplasticizers.

Suitable additives for improving the dimensional stability of thephotographic element are e.g. dispersions of a water-soluble or hardlysoluble synthetic polymer e.g. polymers of alkyl (meth)acrylates,alkoxy(meth)acrylates, glycidyl (meth)acrylates, (meth)acrylamides,vinyl esters, acrylonitriles, olefins, and styrenes, or copolymers ofthe above with acrylic acids, methacrylic acids, Alpha-Beta-unsaturateddicarboxylic acids, hydroxyalkyl (meth)acrylates, sulphoalkyl(meth)acrylates, and styrene sulphonic acids.

According to another embodiment of the present invention a lithographicprinting plate can be obtained by means of the DTR-process using animaging element comprising in the order given a grained and anodizedaluminium support, an optional layer of physical development nuclei anda silver halide emulsion layer. The imaging element of the presentembodiment may be imaged using a camera-exposure or a scanning exposureas described above followed by a development step in the presence ofdevelopment agent(s) and silver halide solvent(s) so that a silver imageis formed in the physical development nuclei layer or directly on thealuminium support. Subsequently the silver halide emulsion layer and anyother optional hydrophilic layers are removed by rinsing the imagedelement with water so that the silver image is exposed. Finally thehydrophobic character of the silver image is preferably improved using afinishing liquid comprising hydrophobizing agents as described above.

The present invention will now be illustrated by way of the followingexamples without however the intention to limit the invention thereto.All parts are by weight unless otherwise specified.

EXAMPLE

Preparation of the silver halide emulsion coating solution.

A silver chlorobromide emulsion composed of 98.2 mole % of chloride and1.8 mole % of bromide was prepared by the double jet precipitationmethod. The average silver halide grain size was 0.4 μm (diameter of asphere with equivalent volume) and contained Rhodium ions as internaldopant. The emulsion was orthochromatically sensitized and stabilized by1-phenyl-5-mercaptotetrazole and a compound of the following formula:##STR16##

A base layer coating solution was prepared having the followingcomposition:

    ______________________________________                                        gelatin           5.5%                                                        carbon black      0.76%                                                       silica particles (5 μm)                                                                      1.6%                                                        ______________________________________                                    

Preparation of the imaging element:

The emulsion coating solution and base layer coating solution weresimultaneously coated by means of the cascade coating technique to apolyethylene terephthalate support provided with a pack of two backinglayers such that the base layer coating was coated directly to the sideof the support opposite to the side containing said backing layers. Theemulsion layer was coated such that the silver halide coverage expressedas AgNO₃ was 1.5 g/m² and the gelatin content was 1.5 g/m². The emulsionlayer further contained 0.15 g/m² of 1-phenyl-4-methyl-3-pyrazolidoneand 0.25 g/m² of hydroquinone. The base layer was coated such that theamount of gelatin in the coated layer was 3 g/m².

The layer nearest to the support of the backing layer pack contained 0.3g/m² of gelatin and 0.5 g/m² of the antistatic agentco(tetraallyloxyethane/methacrylate/acrylic acid-K-salt) polymer. Thesecond backing layer contained 4 g/m² of gelatin, 0.15 g/m² of a mattingagent consisting of transparent spherical polymeric beads of 3 micronaverage diameter according to EP 0080225, 0.05 g/m² of hardening agenttriacrylformal and 0.021 g/m² of wetting agent F₁₅ C₇ --COONH₄.

The thus obtained element was dried and subjected to a temperature of40° C. for 5 days and then the emulsion layer was overcoated with alayer containing PdS as physical development nuclei, hydroquinone at 0.4g/m² and formaldehyde at 100 mg/m².

The following processing solutions were prepared :

    ______________________________________                                        Transfer developer                                                            sodium hydroxide (g)     30                                                   sodium sulphite anh. (g)                                                                               35                                                   compound no 9 of table 2 (g)                                                                           1.2                                                  2-mercapto-5-n.heptyl-  350                                                   oxa-3,4-diazole (mg)                                                          EDTA sodium salt (g)     1                                                    amino-ethyl-amino-ethanol (ml)                                                                         40                                                   methylhydroquinone (g)   2                                                    water to make            1 liter                                              Stabilising liquid                                                            sodium dihydrogenphosphate.H.sub.2 O                                                                   40 g                                                 tri-ethanolamine         10 g                                                 pluronic                 50 mg                                                mercapto-compound       (see table 4)                                         water to make            1 l                                                  ______________________________________                                    

Imaging elements prepared as described above were image-wise exposedusing a camera and were subsequently developed by guiding the imagingelement through the above transfer developer in 20s, 13s or 5s at atemperature of 30° C. The obtained plates were subsequently stabilisedusing the above stabilising liquid.

The obtained printing plates were then mounted on an offset press GTO 46running a K+E 125 ink and a fountain liquid containing 5% of G648(commercially available from Agfa-Gevaert NV) and 15% of isopropanol.The results obtained were as follows:

                  TABLE 4                                                         ______________________________________                                        mercapto-                                                                     compound                                                                      no.    amount    developing                                                                              printing endurance                                                                       ink                                     (table 1)                                                                            (mmol/l)  time (s)  fresh  7 days                                                                              accept.*                              ______________________________________                                        --     --         5        >10000 <1000 <25                                   --     --        13        >10000 <1000 <25                                   --     --        20        >10000 >10000                                                                              <25                                   C      2          5        >10000 >10000                                                                              25-50                                 C      2         13        >10000 >10000                                                                              25-50                                 6      2.4        5        >10000 >10000                                                                              25-35                                 6      2.4       13        >10000 >10000                                                                              25-35                                 2      1.5        5        >10000 >10000                                                                              <25                                   2      1.5       13        >10000 >10000                                                                              <25                                   2/6    2.4/1.5    5        >10000 >10000                                                                              <25                                   2/6    2.4/1.5   13        >10000 >10000                                                                              <25                                   3      0.9        5        >10000 >10000                                                                              <25                                   3      0.9       13        >10000 >10000                                                                              <25                                   4      2.7        5        >10000 >10000                                                                              <25                                   4      2.7       13        >10000 >10000                                                                              <25                                   ______________________________________                                         *the ink acceptance is evaluated as the number of copies that have to be      disposed of before a satisfactory print is obtained.                          mercaptocompound C = 2mercapto-5-n.heptyl oxa3,4-diazole (pK.sub.a = 4.8)

We claim:
 1. A method for making a lithographic printing plate accordingto the silver salt diffusion transfer process using a photographicmaterial comprising on a support a silver halide emulsion layer and alayer containing physical development nuclei comprising the stepsof:image-wise exposing said photographic material; developing a thusobtained image-wise exposed photographic material by guiding saidimage-wise exposed photographic material through an alkaline processingliquid comprising a hydrophobizing agent, in 15s or less and; directlythereafter guiding a thus obtained developed photographic materialthrough a stabilising liquid wherein said stabilising liquid has a pH offrom 5 to 7 and comprises a buffer; characterized in that saidstabilising liquid comprises a compound having a mercapto-group having apH_(a) of 4.5 or less and corresponding to the following formula:##STR17## wherein Z represents an alkyl, an aryl, an alkenyl, an alkynylor a heterocyclic ring.
 2. A method according to claim 1 wherein saidimage-wise exposed photographic material is guided through said alkalineprocessing liquid in 10s or less.
 3. A method according to claim 1wherein the alkaline processing solution further contains a meso-ioniccompound.
 4. A method according to claim 3 wherein said alkalineprocessing solution further contains an alkanolamine.
 5. A methodaccording to claim 3 wherein said meso-ionic compound is a triazoliumthiolate.
 6. A method according to claim 1 wherein said compound havinga mercapto-group having a pK_(a) of 4.5 or less is a hydrophobizingcompound.